So-called confocal optical systems are apparatus for measuring distances. FIG. 22 illustrates the principle of a confocal optical system.
In FIG. 22, light from a light source 1 is condensed by a lens 12 and then directed toward a half-mirror 31 via a pinhole PH1 located at a focal point F1. The light from the light source 1 is transformed by the pinhole PH1 located at the focal point F1 into light equivalent to the a point source. Light reflected from the half-mirror 31 is condensed by a lens 8 and projected onto the surface of an object 9. Shown here is a case in which the surface of the object 9 is in the focal position F2 of the lens 8, and the object 9 is moved and scanned in the X-Y-Z direction by a three-dimensional moving stage 40. Light scattered on the surface of the object 9 passes through the lens 8, travels through the half-mirror 31, and converges toward a point F3 conjugate with the focal position of the light source 1. A pinhole PH2 is located in the position of this focal point F3, and transmitted light is detected by a light sensor 10.
With this structure, the focal point F3 on the conjugate point side moves when the object surface Z0 is shifted to the front or back (Z1 or Z2) of the focal position F2, as shown in FIGS. 23a and 23b, and the output of the light sensor 10 is markedly reduced by the action of the pinhole. FIG. 24 illustrates the relation between the position of the object surface and the output of the light sensor 10.
This structure makes it possible to shift the measurement object 9 by the three-dimensional moving stage 40 in the direction of the Z axis (in the direction of the optical axis) for each X-Y coordinate position, to sample the output of the light sensor 10 in the course of this displacement, and to designate the detected Z position corresponding to the maximum sampling output as the surface position of the object 9. It is therefore possible to subject the measurement object 9 to three-dimensional measurements by sequentially changing the X-Y coordinate position and performing the same measurements.
A disadvantage of this conventional apparatus, however, is that each measurement instant yields information about a single point in space, making it necessary to spend much time to detect the surface shape.
In view of this, an attempt was made in Japanese Laid-Open Patent Application 4-265918 to arrange the confocal optical system in two dimensions and to detect each object position in parallel; the corresponding structure is illustrated in FIG. 25.
Specifically, with the apparatus illustrated in FIG. 25 above, light from a light source i passes through lenses 12 and 2, becomes parallel light, and enters a pinhole array PHA1. The pinhole array PHA1 consists of pinholes arranged in a matrix. The light that has passed through the pinhole array PHA1 is transmitted through a half-mirror condensed by lenses 8a and 8b, and projected onto a measurement object 9. The measurement object 9 is placed on top of a moving stage 35 capable of displacement in the direction of the Z axis. The light reflected by the measurement object 9 is condensed by the lenses 8a and 8b, reflected by the half-mirror 31, and imaged at a position conjugate with the pinhole array PHA1. A pinhole array PHA2 is located in the imaging position, and the light passing through the pinholes is detected by the individual light sensors 10 of a light sensor array.
This conventional structure makes it possible to separately sample the outputs of the individual light Sensors 10 of the light sensor array while displacing the moving stage 35 in the direction of the Z axis, and to designate the detected Z-direction position corresponding to the maximum output of the individual light sensors as the surface position of the object 9.
Because this conventional technique dispenses with the need to move the moving stage in the XY direction, the measuring time can be reduced in comparison with the conventional mechanism illustrated in FIG. 22 above.
Even with the apparatus illustrated in FIG. 25, the moving stage 35 must be moved at a higher speed to further reduce the shape measurement time, but because the moving stage 35 must carry a measurement object, there is a limit to the high-speed movement. In other words, it is very difficult, for example, to use a high-speed moving stage to move a measurement object that is very heavy, a measurement object that has a very fine structure and thus cannot withstand the inertial force created by high-speed displacement, and the like.
Fixing the measurement object 9 and moving and displacing the measuring instrument itself in the Z direction was suggested as a way of overcoming this shortcoming. To be able to be displaced at a high speed, however, the measuring instrument itself must be small and lightweight, have a sturdy structure, and withstand the inertial force created by high-speed displacement. With the conventional technique illustrated in FIG. 25, however, no measures related to high-speed movement have been adopted, thus making it likely that the confocal optical system will break or that other undesirable phenomena will occur during high-speed movement. In particular, it is important for a confocal optical system that the pinhole arrays PHA1 and PHA2 always remain in exact conjugate positions with respect to the half-mirror 31 so that the confocal optical system is always effective, but in the past there has been a limit to reconciling this requirement with a need for a reduction in size and weight.
In addition, a prismatic device was usually used as the half-mirror 31 in FIG. 25, and because this prismatic device requires a cubic space, the focal position of the light source (pinhole array PHA1) and the light-receiving. focal point (pinhole array PHA2) have to be located outside of the cubic space containing the half-mirror 31, so it is impossible to make the distance between the lens 8a and each of the focal points smaller than the length of the cube of the half-mirror 31. In addition, the focal point of the light source of a confocal unit and the light-receiving focal point are located with respect to the half-mirror 31 at geometric distances that vary with each confocal unit. Thus, the conventional mechanism illustrated in FIG. 25 has limits as to the reductions in size and weight, and requires precise alignment between the focal point of the light source and the light-receiving focal point.
An object of this invention, which was devised in view of the foregoing, is to provide a confocal optical apparatus in which the size and weight can be reduced, which allows three-dimensional shape measurements to be performed rapidly and accurately, and in which the alignment of each portion is facilitated.